Familial dilated cardiomyopathy (DCM) is a condition that leads to heart failure and is the most common cause of heart transplantation. The molecular mechanisms of disease progression are not well characterized, and there are no current therapies available for this condition. A characteristic molecular change in DCM is decreased myofilament response to Ca2+. Thus, we hypothesized that by increasing the myofilament response to Ca2+ we may be able to prevent the development of DCM. As proof of principle, we crossed a transgenic model with a missense mutation on sarcomeric protein, tropomyosin (TG-Tm-E54K), which displays a phenotype similar to human DCM, with a transgenic mouse characterized by a constitutive increase in Ca2+-sensitivity. In these double transgenic mice, DCM does not develop. Results from our preliminary studies indicate that novel beta-arrestin signaling pathways are directly involved in signaling to myofilaments, imparting Ca2+-sensitizing post-translational modification, and improving heart function. In this proposal, we hypothesize that beta-arrestin signaling prevents the progression of and reverses DCM by improving the Ca2+ responsiveness of the myofilament. Our approach is to employ TG-TmE54K mice and treat them with the biased ligand, TRV120023, which acts as an angiotensin receptor blocker but is able to activate beta-arrestin signaling, or Losartan, an angiotensin receptor blocker. In Specific Aim 1 we will evaluate the progression of the disease before, during and after treatment in vivo utilizing serial echocardiography. We will also assess left ventricular function in vivo and morphology using hemodynamic studies and histological methods, respectively. To approach Specific Aim 2, we will employ detergent extracted fiber bundles to measure steady-state force and kinetics of treated animals and saline controls. We will also determine the mechanism behind these changes using biochemical methods where we will elucidate the signal transduction pathways activated. In Specific Aim 3 we will use biochemical methods and microscopy to examine beta-arrestin localization to fully characterize these novel signal transduction pathways. We expect that activation of beta-arrestin signaling, while blocking G- protein mediated signaling, will prevent the progression of and reverse DCM by increasing Ca2+-responsiveness of the myofilament. Therefore, results from this proposal will yield pre-clinical data for a potential treatment for DCM, as well as insight into the molecular mechanisms of heart failure.